Question

Which of the following molecules is chiral? How is this determined?

3,4-dimethyl-1-pentene

2-bromobutanamide

2-bromo-1-butanol

chlorocyclohexane

cis-1,2-dimethylcyclobutane

trans-1,2-dimethylcyclobutane

1-pentanenitrile

3-pentanamine

2-aminopropanoic acid (common name: alanine)

2,3-dibromobutane

Answer 1

WARNING! Long answer. You look for chiral carbons and internal planes of symmetry.

Explanation:

A chiral carbon is a carbon with four different groups attached.

An internal plane of symmetry is a plane that divides the molecule that are mirror images of each other.

3,4-dimethylpent-1-ene

Two chiral carbons, no plane of symmetry. ∴ Chiral.

2-Bromobutanamide

One chiral carbon, no plane of symmetry. ∴ Chiral.

2-Bromobutan-1-ol

One chiral carbon, no plane of symmetry. ∴ Chiral.

Chlorocyclohexane

No chiral carbon. ∴ Not chiral.

cis-1,2-Dimethylcyclobutane

Two chiral carbons, but an internal plane of symmetry. ∴ Not chiral.

trans-1,2-Dimethylcyclobutane

Two chiral carbons, no internal plane of symmetry. ∴ Chiral.

Pentanenitrile

No chiral carbons. ∴ Not chiral.

Pentan-3-amine

No chiral carbons. ∴ Not chiral.

2-Aminopropanoic acid

One chiral carbon, no internal plane of symmetry. ∴ Chiral.

2,3-Dibromobutane

This has both chiral and achiral forms.

Two chiral carbons, but an internal plane of symmetry. ∴ Not chiral.

Two chiral carbons, no internal plane of symmetry. ∴ Chiral.